What are the dangers of Japanese knotweed?

Image by Angus MacAskill; Flickr.

Japanese knotweed is a problematic weed that grows approximately 3 metres (10 feet) high and has reddish purple speckled stems and stalks. The leaves of the plant are broad, and they are a light green colour underneath. Japanese knotweed produces white flowers from August until October.

Aside from its original native habitat in Japan, where it grows as an ornamental plant, it is a fast-growing invasive species and gardeners struggle with its expensive removal.


In the United Kingdom, it is against the law to encourage Japanese knotweed growth and propagation. Civil lawsuits typically arise from damages caused by the spread of the plant from one person’s property on to adjoining plots. Those who ignore Japanese knotweed laws face potential fines or even imprisonment.


Serious damage can result from the growth of Japanese knotweed. The plant grows through paving and tarmac areas and damages drains and sewer systems. Flood defence structures commonly incur damage from Japanese knotweed, causing safety issues by preventing the channels from effectively carrying excess floodwater away from affected residential sites. Japanese knotweed also subjects archaeological sites to damage and has the capacity to grow through brick, concrete, wall structures and building foundations. Presence of the plant often results in decreased land and property values. The plant dominates and cuts out native vegetation, in turn affecting the insects and animal species that rely on these plants for food and shelter. Dangerous overgrowth also impedes travel routes for people and animals, hindering natural migration patterns for many species.


Japanese knotweed can grow up to nearly 10 cm (4 inches) a day. One of the main reasons it is so invasive is because it has a vast underground stem system that can branch off nearly 7 metres (23 feet) from the host plant. The plant can regenerate and propagate new plants from even the smallest stem fragments. Stem segments will survive for three years, so complete elimination is crucial in removal efforts. Burning and chemical treatments are the preferred methods used by most experts. The plant can spread easily through the transport of soil and water.